As a method of generating terahertz band (0.3˜3 THz) radiation light based on an optical technique, a device has been conventionally used that mainly uses optical nonlinear material or optical electricity-conducting material. Optical electricity-conducting material and photodiode are both one type of a light-receiving device. Recently, attention has been paid on a photomixing method using photodiode (the conversion of optical beat to an electric signal) because of the high efficiency. With regard to a case using photodiode operating at a communication waveband (wavelength of 1.5 gm), the antenna radiation of a few μW level at 1 THz has been already reported and has been considered as a promising technique. The use of a high-performance photodiode operating at a THz band provides a remarkably-improved spectroscopic performance than in the case where a conventional light-receiving device is used. As a result, an application has been expected for example in which a substance-specific absorption spectrum called as a fingerprint spectrum is observed so that a lot of substances (e.g., medical agents) can be determined.
However, the conventional photodiode structure developed for the purpose of application for an optical communication or a measurement instrument device (having a bandwidth up to 100 GHz at the maximum) is manufactured so as to have a response characteristic having a wide bandwidth suitable for digital application. Thus, the conventional photodiode is not always suitable for the THz band operation. The reason is that, while a typical conventional photodiode has an operation frequency range of about 3 dB bandwidth, in a THz application a wide frequency band including a frequency range much higher than a 3 dB bandwidth is typically used.
The photodiode true response is obtained by a delay id caused by the transit time of electrons or the transit time of both of electrons and holes among carriers (electrons and holes) generated in a light absorption layer. Generally, id can be reduced by reducing the thickness of a depletion layer. This reduction of the thickness of the depletion layer is still one factor required for a higher speed of transit. However, since the reduction of the thickness of the depletion layer requires an increase of the junction capacitance, the photodiode junction size or operation current must be adjusted for optimization. No detailed result has been reported with regard to the photodiode operation focusing on the THz operation as described above.
Furthermore, in the case of a communication photodiode, not only a bandwidth but also a light-receiving sensitivity are considered important. On the other hand, there is a difference in requirement such as that the maximum output is regarded as an important in a THz application. Thus, requirements are different. Although in the THz application, a light-receiving sensitivity having a fixed level or more must be secured, its importance is low. Generally, the operation current is maximized in order to increase the THz output, thus causing a disadvantage of the self heating of the photodiode. Under such conditions, how much THz output can be taken out with regard to fixed operation current is most important than the light-receiving sensitivity. No specific report has been found with regard to the design of a THz photodiode from the viewpoint as described above.